Caster Strut, Wheelchair Frame and Wheelchair

ABSTRACT

A wheelchair has a frame including left and right frame members, each frame member having a first leg configured to support a seat, and a second leg configured to support front caster wheels, the first leg extending substantially horizontally and the second leg extending substantially downward from the first leg. The wheelchair also has a caster strut attached to the second leg of each frame member. The caster strut has first and second strut sections. The first strut section is fixed to the second leg and arranged substantially horizontally. The second strut section is arranged substantially vertically and provides, at a lower end, a mounting for the caster wheel. The caster strut further contains a curved section intermediate the first and second strut sections.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/008,806, filed Dec. 21, 2007, and entitledLIGHTWEIGHT WHEELCHAIR, and PCT application, entitled CASTER STRUT,WHEELCHAIR FRAME AND WHEELCHAIR. This application is the National Phaseof International Application PCT/EP2008/011015 filed Dec. 22, 2008 whichdesignated the U.S. and that International Application was published inEnglish under PCT Article 21(2) on Jul. 2, 2009 as InternationalPublication Number WO 2009/080346. PCT/EP2008/011015 claims priority toU.S. Provisional Application No. 61/008,806, filed Dec. 21, 2007. Thus,the subject nonprovisional application claims priority to U.S.Provisional Application No. 61/008,806, filed Dec. 21, 2007. Thedisclosures of both applications are incorporated herein by reference.

TECHNICAL FIELD

This invention relates to lightweight wheelchairs, caster struts forwheelchairs, and frames for wheelchairs.

BACKGROUND OF THE INVENTION

Manual wheelchairs are comprised of a wheelchair frame that is supportedon the ground by two front caster wheels and two rear drive wheels. Thedrive wheels are large relative to the front caster wheels. The framesupports a seat for the wheelchair user. In lightweight wheelchairs, thedrive wheels are configured to allow the user to propel and steer thewheelchair by rotating the large rear drive wheels by hand. Lightweightwheelchairs are usually configured to be folded and disassembled forease of transport.

Wheelchair frames for manual wheelchairs are commonly made of benttubing of aluminum or other metals or alloys. At the front of thewheelchair frame, the tubing is often configured to accommodate thewheelchair user's feet and a footrest The tubing also provides asuitable structure to mount the front caster wheels. It would beadvantageous if the arrangement of the front portion of frames forwheelchairs could be improved.

SUMMARY OF THE INVENTION

According to this invention, there is provided a wheelchair having aframe including left and right frame members, each frame member having afirst leg configured to support a seat, and a second leg configured tosupport front caster wheels, the first leg extending substantiallyhorizontally and the second leg extending substantially downward fromthe first leg. The wheelchair also has a caster strut attached to thesecond leg, the caster strut having first and second strut sections, thefirst strut section being fixed to the second leg and arrangedsubstantially horizontally, and the second strut section being arrangedsubstantially vertically and providing at its lower end a mounting forthe caster wheel. The caster strut further contains a curved sectionintermediate the first and second strut sections.

According to this invention, there is also provided a wheelchair havinga frame including left and right frame members, each frame member havinga first leg configured to support a seat, and a second leg configured tosupport front caster wheels. The first leg extends substantiallyhorizontally and the second leg extends substantially downward from thefirst leg. The wheelchair also has a caster strut attached to the secondleg, the caster strut having first and second strut sections and acurved section. The curved section is intermediate the first and secondstrut sections. The first strut section is attached to the second leg.The cross-section of the first strut section is larger than thecross-section of the second strut section.

According to this invention, there is also provided a wheelchair havinga frame including left and right frame members. Each frame member has afirst leg configured to support a seat and a second leg configured tosupport front caster wheels. The first leg extends substantiallyhorizontally and the second leg extends substantially downward from thefirst leg. The wheelchair also has a caster strut attached to the secondleg. The caster strut has first and second strut sections and a curvedsection intermediate the first and second strut sections, the firststrut section being attached to the second leg. The first strut sectionhas a varying cross-sectional area with the largest cross-sectional areaat the point where the first section is attached to the second leg. Thecross-sectional area of the first section tapers to a smallercross-sectional area as the distance from the second leg increases.

Various advantages of this invention will become apparent to thoseskilled in the art from the following detailed description of thepreferred embodiment, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wheelchair, with the seat, backrest,and one of the drive wheels partially removed for clarity.

FIG. 2 is a side view of the wheelchair frame.

FIG. 3 is a perspective view of the wheelchair frame and backrest frame.

FIG. 4 is a view of an end of a caster strut for attachment to membersof the wheelchair frame.

FIG. 5 is a perspective view of the caster strut of FIG. 4.

FIG. 6 is a perspective view from another angle of the caster strut ofFIGS. 4 and 5.

FIG. 7 is a side view of a tubular structure for forming the casterstrut at an intermediate stage of production.

DETAILED DESCRIPTION OF THE INVENTION

The lightweight wheelchair described in this specification andillustrated in the accompanying drawings includes a caster strut that isconfigured to make the wheelchair easier to handle. This is particularlytrue when the wheelchair is being transported. The caster strut issufficiently strong to be able to support the weight of the wheelchairand the user. In one embodiment, the caster struts allow the casterwheels to be placed closer to the rear wheels, thereby improving themaneuverability of the wheelchair.

Referring to FIGS. 1-3, a wheelchair 1, shown here by way of example, issupported by left and right rear main wheels 2, 3 and left and rightcaster wheel assemblies 4, 5, comprising caster wheels 6, 7. Thewheelchair 1 includes a base frame, shown generally at 8 a, and abackrest frame, shown generally at 8 b.

As shown in FIG. 3, the base frame 8 a includes left and right sideframe members 9, 10 arranged on opposite sides of a longitudinalwheelchair central axis 48, and aligned with a direction “d” of forwarddisplacement, or forward direction of the wheelchair 1. The left andright side frame members 9, 10 are generally L-shaped. In other words,the wheelchair 1 has an open frame. The sideframe members 9, 10 eachinclude a longitudinally extending first leg 11, 12 and a downwardlyextending second leg 14. Each first leg 11, 12 transitions into a secondleg via a respective curved section 15, 16. The first legs 11, 12 extendgenerally horizontally, and the second legs 13, 14 depend downwardly andforwardly. It is noted that the angle of the first legs 11, 12 to thehorizontal can be adjusted by adjusting the height of the first legs 11,12 relative to the ground. They are thus only predominantly oriented ina horizontal direction, and but not exclusively oriented that way.Additionally, the directional labels such as downwardly, forwardly,horizontally and the like are for description purposes relative to thedrawings and not limitations to the scope of the invention.

The first legs 11, 12 are arranged for supporting a seat (not shown) ofthe wheelchair 1. In particular, a seat sling (not shown) can be slungbetween the first legs 11, 12 of the sideframe members 9, 10, on top ofwhich a seat cushion (not shown) of any suitable shape or configurationmay be placed.

The curved sections 15, 16 of the sideframe members 9, 10 are curved inmultiple planes, such that the second legs 13, 14 are separated fromeach other by a shorter distance than the first legs 11, 12 of thesideframe members 9, 10. Thus, the seat can be relatively wide, whereasthe second legs 13, 14 of the sideframe members 9, 10 contribute toproviding support for the wheelchair occupant's legs. The second legs13, 14 have longitudinal axes 50, 52, as shown in FIG. 3.

The second legs 13, 14 are supported by the caster wheel assemblies 4, 5via caster struts 17, 18 in which the caster wheel assemblies 4, 5 arepartially accommodated. As shown in FIG. 3, the caster struts 17, 18 areattached at respective longitudinal ends 17 a, 18 a (18 a is not shownin FIG. 1, for clarity) to the sides of the respective second legs 13,14 at a position spaced above the lower ends of the second legs 13, 14.The caster struts 17, 18 place the caster wheels 6, 7 at a widerdistance from the longitudinal wheelchair central axis 48 of thewheelchair 1 when compared to the second legs 13, 14, to providestability. The caster struts 17, 18 are also attached to second legs 13,14 of the sideframe members 9, 10 at a slightly rearward angle withrespect to the second legs 13, 14 so as to be oriented at least partlyin a direction opposite to the direction d of displacement of thewheelchair 1, i.e. closer to the rear wheels 2, 3. This improves themaneuverability of the wheelchair 1 by shortening the wheelbase. In oneembodiment, the amount of the rearward directional component relative toa line perpendicular to the central longitudinal axis 48 is within therange of from about 5 degrees to about 40 degrees. In anotherembodiment, the rearward directional component is within the range offrom about 15 degrees to about 25 degrees. Other angles of orientationmay be used.

Because the wheelchair 1 has an open frame, it is supported by thecaster wheels 6, 7 only via the caster struts 17, 18 and second legs 13,14. There is no other connection between the caster wheel assemblies 4,5 and the first legs 11, 12 of the sideframe members 9, 10. Similarly,the caster struts 17, 18 form the only connections between the casterwheel assemblies 4, 5 and the sideframe members 9, 10.

In one embodiment, the rear wheels 2, 3 are positioned as close togetheras possible, and in particular, as close to the sideframe members 9, 10as possible. The seat should be wide enough to accommodate an occupantcomfortably, but, overall, the wheelchair 1 should be narrow for bettermaneuverability and access to buildings, transport means and the like.This is only possible if the set camber of the rear wheels 2, 3 ismaintained when the wheelchair 1 is occupied. There must be no “sagging”of the frame, and in particular, no rotation or torsion of the sideframemembers 9, 10 under the weight of the occupant. Otherwise, the wheels 2,3 would angle inwards at the top towards the first legs 11, 12 and runagainst them or against side guards (not shown) mounted to the firstlegs 11, 12 of the sideframe members 9, 10. To prevent such sagging, andto provide rigidity for the wheelchair base frame 8 a, the first legs11, 12 are directly connected by a cross-brace 19 and indirectly by anaxle tube 20. Additionally, a foot rest assembly 24 a with left andright foot rest frame members 21, 22, in general alignment with andconnected to the second legs 13, 14, includes at least one cross-member23 interconnecting the left and right foot rest frame members 21, 22. Inother embodiments, the cross-member 23 is eliminated and instead adifferent cross-member (not shown) may be situated below the foot plate24.

For adjustment of the height of the frame 8 a relative to the foot plate24 it is contemplated that the left and right foot rest frame members21, 22 of the foot rest assembly 24 a be accommodated within the secondlegs 13, 14 in telescoping manner. Other methods of connecting the footrest frame members 21, 22 to the second legs 13, 14 may be used. Thetelescopic movement of the left and right foot rest frame members 21, 22can be fixed in a desired position by any suitable means. In oneembodiment, the foot rest frame members 21, 22 can be fixed within thesecond legs 13, 14 in one of a number of positions, and secured inplace, for example, by biased pins in the foot rest frame members 21,22, arranged to cooperate with any of a series of holes in the secondlegs 13, 14 of the sideframe members 9, 10.

The first legs 11, 12 of the sideframe members 9, 10 are each linked tothe axle tube by a respective axle plate 25, 26 to which a respectiveaxle tube clamp 27, 28 is connected in one of a number of pre-determinedpositions on the axle plate. The interconnection between the axle tubeclamp 27, 28 and the axle plate 25, 26 allows for movement of the axletube 20 between the different pre-determined positions at varyingdistances relative to the first legs 11, 12 of the sideframe members 9,10. The positions are at varying distances to the seat. In this manner,the rear seat height can be adjusted because the axle tube 20accommodates camber tubes (not shown in detail) for holding axles of therear wheels 2, 3.

At least one of the camber tube and the axle is removable from the axletube 20 so that the rear wheels 2, 3 can be taken off the wheelchairframe 8 a when the wheelchair 1 needs to be transported, as is shown inFIG. 2.

The back rest frame 8 b is pivotably connected to the base frame 8 a bya connection mechanism comprising, in this example, left and right angleplates 31, 32, that enables left and right backrest frame members 28, 29to be fixed at any of several angles relative to the first legs 11, 12of the sideframe members 9, 10. In particular, as shown in FIG. 2, thebackrest frame members 28, 29 can be folded and fixed in a position inwhich their longitudinal central axes are predominantly aligned withlongitudinal central axes of the first legs 11, 12 of the sideframemembers 9, 10. In this configuration, the wheelchair 1 can betransported easily, e.g. in the trunk of a car. Optionally, thewheelchair 1 can be carried with one hand by the cross brace 19 betweenthe sideframe members 9, 10 or a similar cross-brace provided betweenthe backrest frame members 28, 29. In the configuration of FIG. 2, thecaster struts 17, 18 are quite exposed. Due to their shape, however,there is reduced risk of their causing damage or injury.

FIGS. 4-6 show the right caster strut 18 by way of example, the othercaster strut 17 being a mirror image. FIG. 7 shows the caster strut 18at an intermediate stage of its production.

The caster strut 18 is a tubular structure with, as viewed along itslongitudinal central axis, a first section 34, a second section 36, anda curved section 35, by way of which the first section 34 transitionsinto the second section 36. The caster strut 18 can be made of compositematerials, metal or a metal alloy. Suitable materials includealuminium-scandium alloys, aluminium alloys from the 7000 series,particularly aluminium 7003, or aluminium from the 6000 series.Aluminium 7000 has a relatively high tensile strength, which allows oneto bend a tubular member into a shape with a curved section 35 having asmall radius of curvature. If made of a composite, the caster strut 18can be chemically bonded to the second leg of the side frame member. Ifmade of metal or a metal alloy, it may alternatively be welded. In aquite different embodiment to the one illustrated herein, a stub (notshown) may be provided on the second leg, and the caster strut 18 may beattached onto the stub, e.g. mechanically. In yet another differentembodiment, cooperating and interlocking attachment means are provided.The illustrated embodiment is relatively lightweight and strong,particularly when welded to the second leg 13, 14 of the side framemember 9, 10. The length of the various sections 34, 35, and 36 mayvary. In one embodiment, the length of the first section 34 is withinthe range of about 1.5 times to about 3 times the length of the secondsection 36. In another embodiment, the length of the first section 34 iswithin the range of about 1.8 times to about 2.2 times the length of thesecond section 36.

Although the first section 34 is substantially horizontal, it may bepositioned at a slight angle to the horizontal, sloping downward as itextends from the second leg 14. In one embodiment, the first section 34has a slope within the range of from about 10 degrees to about 20degrees to the horizontal.

With reference to FIGS. 4-6, the end 18 a of the first section 34, atwhich the caster strut 18 terminates, is provided with upper and lowercut-outs 37, 38, that are adapted to the profile of the second leg,which is partially received in the cut-outs 37, 38. When attached, thesecond leg 14 is thus partially enveloped by the end 18 a of the casterstrut 18. As a result, there is a relatively large contact area betweenthe caster strut 18 and the second leg 14. This relatively large contactarea facilitates bonding, particularly by welding, of the caster strut18 to the second leg 14 along the edges of the cut-outs 37, 38. The areais also relatively large because the first section 34 of the casterstrut 18 has a substantially oval cross-sectional shape, as shown inFIG. 4. In alternative embodiments, the cross-section can be teardropshaped or T-shaped. The oval shape can be produced from cylindricaltubing having a circular cross section.

Referring to FIG. 4 in particular, the end 18 a of the caster strut 18,where the first section 34 terminates, is shaped (by way of the cut-outs37, 38 in this case) to attach to the second leg 14 with a certainorientation relative to the longitudinal axis 52 of the second leg 14.The oval cross-section of the first section 34 has a largest diameter ordimension “A” in a direction along a major axis 39, which ispredominantly parallel to the longitudinal axis 52 of the second leg 14when attached. Thus, no separate buttresses, fins, or similarstrengthening support structures are required between an underside 40 ofthe first section 34 and the second leg 14 of the side frame member 10.

The illustrated caster strut 18 may be manufactured by starting with around tube, reducing the tube diameter, at least at an end 41 that willbecome the lower end of the second section 36, bending the tube toprovide a bent section corresponding to the curved section 35, andadjusting the cross-sectional shape (including the diameter) of thefirst section 34 to give it a generally oval-shaped end. The adjustmentcan be carried out using the technique of hydroforming to widen thefirst section 34 and give it an oval shape. Alternatively, if lesswidening is required, a circular precursor of the first section 34 cansimply be pressed from one side. The result of these steps is shown inFIG. 7. Subsequently, the cut-outs 37, 38 are provided, as well asapertures 42, 43 and a slot 44, the purpose of which will be explainedbelow. Other manufacturing techniques may be used to manufacture thecaster strut 18. In another embodiment, the thickness of the tube variesalong a portion of the first section 34.

Referring in particular to FIG. 7, it can be seen that, in alongitudinal cross-sectional plane parallel to the plane in which thecurve of the curved section 35 lies, the first section 34 tapers towardsthe curved section 35. The major diameter A, shown in FIG. 4, of theoval cross-section at a first position P1, shown in FIG. 7, near the end18 a of the first section 34 is larger than the major diameter of theoval cross-section at a second point P2 nearer the curved section 35. Inthe illustrated embodiment, a minor diameter “B”, shown in FIG. 4,remains generally the same along the length of at least the firstsection 34 of the caster strut 18. In other embodiments, the minordiameter B may vary. Therefore, in one embodiment of the caster strut18, the first strut section 34 has a varying cross-sectional area withthe largest cross-sectional area at the point P1 where the first section34 is attached to the second leg 14, and with the cross-sectional of thefirst section 34 tapering to a smaller cross-sectional area as thedistance from the second leg 14 increases.

A radius of curvature, “R”, of the center line of the curved section 35is smaller than or equal to four times the diameter of the caster strut18 at a transition from the first section 34 to the curved section 35.More particularly, radius R is equal to or smaller than three times thediameter. The taper between P1 and P2 is provided to make this radius Rsmaller in absolute terms and still provide sufficient stiffness. Theradius of curvature R cannot generally be made very small in relativeterms, i.e. relative to the diameter of the caster strut 18 in the planeof the curve, because such a curve cannot be provided by bending withoutrisk of tearing the tubular structure. To reduce this risk, the bend canbe made with the tubular structure at a temperature corresponding to agenerally ductile, or almost ductile, state of the material of the tubeforming caster strut 18.

In contrast to the first section 34 of the caster strut 18, the secondsection 36 has a generally circular cross-section, at least at the lowerend of the caster strut 18 where it provides an aperture for insertingpart of the caster wheel assembly 5.

It will be appreciated from the description provided above that thefirst, second and curved sections, 34, 35, and 36 respectively, arecontinuations in the longitudinal direction of each other, so that theyare sections of a single tubular member. Thus, there are no jointsbetween the sections that need to be strengthened.

The second section 36 of the caster strut 18 has a longitudinal axis 45.In one embodiment, the caster wheel assembly 5 will be at least partlyhoused in lower end 41 of the second section 36 of the caster strut 18.Caster wheel assemblies 4, 5 are known in the art. Such a caster wheelassembly 4, 5 includes a swivel post (not shown) suspended on a pivotaxle that is inserted through the apertures 42, 43 about which, when inuse, the swivel post can rotate to at least a limited extent. Thus, whenthe rear seat height above ground is adjusted by displacing the axletube clamp 27 along the axle plate 25, 26, the angle of the swivel postrelative to a longitudinal central axis 45 can be adjusted, so that theswivel post remains generally aligned with the vertical.

The slot 44 is provided to serve as a guide for guiding movement of amember attached to the swivel post of the caster wheel assembly 5, andis oriented to guide the member along a trajectory around thelongitudinal central axis 45 of the second section 36. Ends 46, 47 ofthe slot 44 limit this trajectory to a length corresponding to arotation of less than 180 degrees, in particular less than 90 degrees.

The caster strut 18 thus facilitates adjustment of the wheelchair 1 tothe needs of the individual user, as well as contributing to therigidity and low weight of the wheelchair frame 8 a. This isparticularly due to the small radius of curvature R of the curvedsection 35 which makes the wheelchair 1 compact when folded and partlydisassembled for transport. In that state, the risk of damage or injuryis kept low by the smooth surface of the caster struts 17, 18.

The cutouts 37, 38 provide the end 18 a of the caster strut 18 with ashape for direct contact with the second leg 14. As can be seen in theembodiment shown in FIG. 6, cutouts 37, 38 do not completely cut backall of the original end of the tube. Instead, there are remainderportions 54, 56, that are not cut out. The cutouts 37, 38, extend aroundthe perimeter of the oval within the range of from about 60 percent toabout 90 percent of the entire perimeter of the oval. In otherembodiments the cutouts 37, 38, extend around the perimeter of the ovalwithin the range of from about 70 percent to about 80 percent of theentire perimeter of the oval. The remainder portions 54, 56 provide astronger connection to the second leg 14 when the caster strut 18 iswelded to the second leg 14. In other embodiments, the reminder portionsare not present. The non-circular cross-section provides additionalstrength in at least one longitudinal cross-sectional plane of thecaster strut 18, e.g. the plane in which bending forces occur. All thisis made possible without resorting to an increase in the diameter of thesecond legs 13, 14, of the left and right frame members 11, 12, thuscontributing to a low overall wheelchair frame weight.

If the end 18 a of the caster strut 18 at which the first longitudinalsection 34 terminates is configured, and in particular shaped, to attachto the second leg 14 with a certain orientation of a longitudinal axis52 of the second leg 14, and the non-circular cross-section has a largerdiameter in a direction predominantly parallel to the longitudinal axis52 of the second leg 14, when attached, then the caster strut 18 isparticularly strengthened in the longitudinal cross-sectional plane ofthe caster strut 18 in which bending forces occur. This plane ispredominantly oriented in the vertical direction when the wheelchair 1is in use. The diameter of the caster strut 18 can be made smaller intransverse longitudinal cross-sectional planes, which keeps the weightdown and allows attachment to the left or right frame member 11, 12,having a second leg 13, 14, of limited diameter.

An advantage of the one-piece caster strut 18 is that it can avoid thesharp edges sometimes present in two-part caster struts. The outsidesurface of the caster strut 18 can be essentially smooth, so that thereis minimized risk of its catching on anything as the wheelchair frame ishandled. The absence of angled joint sections (e.g. T-pieces) betweenthe first and second longitudinal sections 34, 36, makes the casterstrut 18 relatively lightweight. By providing a caster strut 18 that isattached with an end 41 to a second leg 14 of one of left and rightframe members 9, 10, arranged on opposite sides of a central axis 48,with each frame member including a first leg 12 for supporting a seatthat transitions into a second leg 14 depending downwardly, thewheelchair frames are more compact when the rear wheels 2, 3 areremoved, and are lighter than those with closed side frames. Because thefirst longitudinal section 34 of the caster strut 18 terminates at anend 41 of the caster strut where the first longitudinal section 34 isattached to the second leg at an angle thereto, the caster struts 17, 18allow the caster wheels 6, 7 to be placed further apart, improvingstability. The second legs 12, 13 of the left and right frame members 9,10 can be placed closer together to support an occupant's legs.Moreover, the caster struts 17, 18 allow the caster wheels 6, 7 to beplaced closer to the rear wheels 2, 3, improving the maneuverability ofthe wheelchair 1.

The principle and mode of operation of this invention have beendescribed in its preferred embodiments. However, it should be noted thatthis invention may be practiced otherwise than as specificallyillustrated and described without departing from its scope.

1-15. (canceled)
 16. A wheelchair having a frame including left andright frame members, each frame member having a first leg configured tosupport a seat, and a second leg configured to support front casterwheels, the first leg extending substantially horizontally and thesecond leg extending substantially downward from the first leg, and acaster strut attached to the second leg, the caster strut having firstand second strut sections, the first strut section being fixed to thesecond leg and arranged substantially horizontally, and the second strutsection being arranged substantially vertically and providing at a lowerend a mounting for the caster wheel, the strut further containing acurved section intermediate the first and second strut sections.
 17. Thewheelchair of claim 16 in which the caster strut has a tubularstructure, with one portion of the caster strut having a cross-sectiondifferent from another portion of the caster strut.
 18. The wheelchairof claim 17 in which the one portion of the caster strut is the firststrut section, and the cross-section is non-circular.
 19. The wheelchairof claim 18 in which the cross-section is oval.
 20. The wheelchair ofclaim 3 in the second leg has a longitudinal cross-sectional, and inwhich the largest diameter of the cross-section is predominantlyparallel to the longitudinal axis of the second leg.
 21. The wheelchairof claim 18 in which the second longitudinal section has a substantiallycircular cross-section.
 22. The wheelchair of claim 17 in which thefirst longitudinal section tapers toward the curved section in at leastone longitudinal cross-sectional plane.
 23. The wheelchair of claim 17in which the caster strut has a caster strut longitudinal end, with thecaster strut being attached to the second leg at the caster strutlongitudinal end of the caster strut.
 24. The wheelchair of claim 17 inwhich the second longitudinal section includes at least one guide forguiding movement of a member attached to a swivel post of the casterwheel assembly, the guide oriented to guide the member along atrajectory around a longitudinal central axis of the second longitudinalsection.
 25. The wheelchair of claim 17 in which each caster strut isattached to a second leg at an angle so as to be oriented at leastpartly in a direction opposite to the forward direction.
 26. Thewheelchair of claim 17 in which the thickness of the tube first sectionvaries along a portion of the first section.
 27. The wheelchair of claim17 in which the cross-sectional shape of the first section varies alonga portion of the first section.
 28. A wheelchair having a frameincluding left and right frame members, each frame member having a firstleg configured to support a seat, and a second leg configured to supportfront caster wheels, the first leg extending substantially horizontallyand the second leg extending substantially downward from the first leg,and a caster strut attached to the second leg, the caster strut havingfirst and second strut sections and a curved section intermediate thefirst and second strut sections, the first strut section being attachedto the second leg, and the cross-section of the first strut sectionbeing larger than the cross-section of the second strut section.
 29. Thewheelchair of claim 28 in which the cross-section of the first andintermediate strut sections is circular.
 30. The wheelchair of claim 28in which the cross-sectional shape of the first section varies along aportion of the first section.
 31. The wheelchair of claim 28 in whichthe cross-section of the firsts section at the point of attachment tothe second leg is oval.
 32. The wheelchair of claim 28 in which thecurved section has a centreline with a radius that is smaller than orequal to four times the diameter of the caster strut at a transitionfrom the first section the curved section.
 33. The wheelchair of claim28 in which the radius is equal to or smaller than three times thediameter.
 34. The wheelchair of claim 28 in which the length of thefirst strut section is within the range of about 1.5 to about 3 timesthe length of the second strut section.
 35. A wheelchair having a frameincluding left and right frame members, each frame member having a firstleg configured to support a seat, and a second leg configured to supportfront caster wheels, the first leg extending substantially horizontallyand the second leg extending substantially downward from the first leg,and a caster strut attached to the second leg, the caster strut havingfirst and second strut sections and a curved section intermediate thefirst and second strut sections, the first strut section being attachedto the second leg, and the first strut section having a varyingcross-sectional area with the largest cross-sectional area at the pointwhere the first section is attached to the second leg, and with thecross-sectional area of the first section tapering to a smallercross-sectional area as the distance from the second leg increases.